Synthesis, Characterization, and Evaluation of the Antifungal Properties of 3-Indolyl-3-Hydroxy Oxindole Derivatives Against Plant Pathogenic Fungi

To discover novel fungicides with good inhibitory effects on plant fungal diseases, twenty-five 3-indolyl-3-hydroxy oxindole derivatives (3a-3y) were synthesized. These newly derivatives were characterized by NMR and HRMS. Their antifungal activities against five plant pathogenic fungi were assessed in vitro. Most of the compounds exhibited moderate to excellent antifungal activities against the five pathogenic fungi. Notably, compounds <b>3t</b>, <b>3u</b>, <b>3v</b>, and <b>3w</b> displayed remarkable and broad-spectrum antifungal activities comparable to or superior to those of the fungicides carvacrol (CA) and phenazine-1-carboxylic acid (PCA). Among them, compound <b>3u</b> displayed the most excellent antifungal activity against <i>Rhizoctonia solani</i> Kühn (<i>R. solani</i>), with an EC₅₀ of 3.44 mg/L, which was superior to CA (7.38 mg/L) and PCA (11.62 mg/L). Preliminary structure-activity relationship (SAR) results indicated that the introduction of I, Cl, or Br substituents at position 5 of the 3-hydroxy-2-oxindole and indole rings is crucial for compounds to exhibit good antifungal activity. The in vivo antifungal activity assay showed that compound 3u has good curative effects against <i>R. solani</i>. The current results suggest that these compounds are capable of serving as promising lead compounds.